Two-component systems based on hydroxylated acrylic resins or copolymers and polyisocyanates are already known, and are generally considered most suitable for the process of painting or shellacking plastic materials.
These systems offer many possibilities of adjusting properties such as hardness, flexibility, chemical and physical resistances, etc. Their low crosslinking or drying temperatures make it possible to use them on plastics sensitive to elevated temperatures.
Hydroxylated acrylic resins or copolymers for painting plastics have hitherto been manufactured by solution polymerization, in liquid form, which makes it necessary for the user to partially or completely apply solvents used in the same polymerization. Many of these solvents favor the formation of microfissures on the surface of the painted or shellaced plastic, because the plastic part is sensitive to or caused to swell by the solvent, which may release internal stresses resulting in the formation of such microfissures which, in their turn, lower the mechanical resistance of the plastic and the paint or shellac used.
The internal stresses of the plastic are those formed during the preparation of parts by injection molding, extrusion, or transformation by heat or pressure with their appropriate final cooling.
Solid acrylic resins or copolymers for paints in powder form to date contain crosslinked carboxyl or glycidyl groups, and they crosslink with oxazoline or anhydrous acids. Results with their use likewise are unsatisfactory.